Why Stable Requirements Are Hard to Achieve

Because requirements inevitably change — customers cannot predict how a system will affect working practices, how it will interact with other systems, and what user operations should be automated until they gain experience with it

Explanation

Initial requirements inevitably change because customers find it impossible to predict how a system will affect working practices, how it will interact with other systems, and what user operations should be automated. It may only be after the system is delivered and users gain experience with it that the real requirements become clear. Even then, requirements are likely to change quickly due to external factors.